1. Field of the Invention
This invention relates to high torque drive systems. The invention has particular applicability to drive systems for vehicle-mounted rotators which carry booms.
2. Description of the Prior Art
There are currently in use many types of vehicles which carry rotatably mounted booms. In most instances the boom is mounted on a pedestal or platform (sometimes called a "rotator") that incorporates a drive system for moving the pedestal arcuately in a horizontal plane relative to the vehicle. Some examples of these are the so-called "cherry-picker" devices (used for raising a man off the ground to enable him to repair power lines, cut tree limbs, etc.), earth-moving equipment such as backhoes, lifting cranes and fire-fighting equipment. In normal use, it is common that the booms are subjected to forces directed transversely with respect to the longitudinal axis of the boom. Many of these booms are extensible and can exceed 75 feet in length when fully extended. Therefore, when a transverse force is applied at the end of the boom, a high torque load is placed on the system which supports and drives the rotator.
One example that illustrates the foregoing is a water tower-type fire truck. In this apparatus, either a telescoping or articulated boom is mounted on a truck chassis by a rotatable pedestal. A liquid extinguishant, commonly water, is pressurized by pumps mounted on the truck and is supplied through the boom to a nozzle mounted on the upper end of the boom. Such nozzles are commonly mounted for rotation relative to the boom in both horizontal and vertical planes. As the jet of water leaves the nozzle, it subjects the nozzle to a reaction force opposite in direction to the direction of the jet. This must be resisted by the boom in order to maintain the nozzle in position. When the nozzle is rotated in a horizontal place so that the longitudinal axis of the nozzle is not coincident with the longitudinal axis of the boom, a turning moment is created by the nozzle reaction force which is applied at the end of a rather long moment arm, namely, the extended boom. Accordingly, it is not unusual for the rotation means for this type of apparatus to be designed to transmit in the order of 300,000 inch-pounds of torque.
In equipment of known design, a large gear is non-rotatably mounted on the vehicle chassis concentrically with the axis of rotation of the rotator. A drive motor is mounted on the rotator and the drive motor drives a pinion which engages the fixed gear. The platform is caused to rotate as the pinion travels about the periphery of the fixed gear. A drawback of this design is that in order to obtain the necessary reduction in drive ratio between the pinion and the ring gear to enable the rotation of the rotator, it is necessary to employ a large, fixed gear and a small pinion. The relatively small pinion is less capable of sustaining the high torque loads produced by forces acting on the boom. A problem that has occurred is that the teeth of the pinion are broken off, and the drive system thereby disabled.
Planetary transmissions comprised of a sun gear, a plurality of planet gears, and an internal ring gear are, of course, known. However, the machining and assembly tolerances required in such transmissions are high. It is especially difficult and costly to achieve these tolerances in large transmissions, such as are necessary for rotating vehicle-mounted booms as herein disclosed. Some efforts have been made to circumvent the tolerance problem by mounting the planet gears on eccentrics or levers to allow radial movement of the planet gears relative to the sun gear and the ring gear. Such systems do not accomodate eccentricities of the ring gear.